1. Field of the Invention
This invention relates to a process for removing roll-set curl from thermoplastic webs, particularly thermoplastic webs used in photographic films. Still more particularly, it relates to a process for removing roll set curl that has developed during storage of photographic film, in rolls, especially medical and industrial x-ray films, prior to cutting into sheets, to provide flat sheets compatible with automatic film changing apparatus.
2. Description of the Prior Art
Curl in finished sheets of industrial and medical X-ray films can be substantially attributed to the storage of photographic material in rolls. The tension and compression of the outer and inner surfaces, respectively, cause elongation and contraction of the surfaces, particularly when the material comprises a substrate with coatings on both sides. Upon unwinding these surfaces do not recover their original size. The problem may be accentuated by hardening of the emulsion and antibrasion layers.
Processes and apparatus for eliminating curl from webs are well known in the web handling art. U.S. Pat. No. 2,658,432 teaches the use of a device for decurling paper webs having spaced ironers and a roll over which the web is looped to bring the web into contact with the ironers and cancel roll-set curl in the web as it passes over the ironers. The device of this patent uses tensioning rolls to increase the decurling action in proportion to the curl of the paper to the web core.
U.S. Pat. No. 3,002,222 teaches heating a thermoplastic web to an empirically determined temperature which renders the film more pliable and applying a reverse curl to remove roll-set curl.
It is well known to remove roll-set curl through the application of reverse curl action with or without heat. The disadvantages in such processes is that the rolls applying the reverse curl often damage the sensitive web surface by introducing scratches, nicks or similar defects incidental to the application of pressure on the surface to generate the reverse curl.
It is also known that heating a thermoplastic material will relieve stresses. Alles, U.S. Pat. No. 2,779,684 teaches a process whereby a polyester film is heat treated to a temperature of 110.degree.C to 150.degree.C for a period of 60 to 300 seconds to produce a film base capable of resisting shrinkage when subsequently exposed to 120.degree.C for a period of up to 5 minutes. Spencer, U.S. Pat. No. 3,526,695 teaches a process where a thermoplastic film is first heated to a temperature of 88.degree.C, to 135.degree. centigrade, and then cooled while supported, to below the second order transition temperature of the base. This treatment reduces corrugation type defects which are generated on the base during manufacturing.
It has now been unexpectedly discovered that heat treating coated or uncoated photographic film base at low temperatures (above the second order transition temperature of the base,) for brief periods of time, e.g., around seven seconds, eliminates, roll-set curl. In the cases where the photographic film base was coated with photosensitive layers, such treatment does not adversely affect the photographic characteristics of the film.
It was further found that subjecting the coated base to 50.degree.C. did not remove curl to any appreciable degree, but increasing the film temperature to just above the second order transition temperature of the base for a brief period and then cutting the web into individual sheets of the commercially desired size and shape, did produce sheets free of roll-set curl.